Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 109

located deeply from the Stern layer. By product analysis using 31 P-NMR, this oximate
was found to hydrolyze (5) to the corresponding o -demethylated derivative via the
SN2 (C) mechanism in addition to the S N 2 (P) reaction. Therefore, (5) is most likely
to be solubilized in the HDTMA + micelles as its phenyl ring is located in the Stern
layer with the P

S moiety protruding toward the micellar surface.
The alkaline hydrolysis rate of carbaryl (8) exhibited the typical dependency of
the HDTMA Br concentration (see Fig. 7a) (Gonzalez et al. 1992). From the tem-
perature dependency of the hydrolysis rate, the activation enthalpy was estimated
to be 16.4 cal/mol

. K, slightly larger than that without micelles, and therefore the
reaction was considered to obey the E1cB mechanism where the formed nitranion
was stabilized by association with the positively charged head groups (Patel and
Wurster 1991). The enhanced alkaline hydrolysis was reported for carbofuran (9)
in the cationic micelles, while at higher concentrations of alkyl sulfates and alkoxy
ethoxylates it was markedly retarded (Arias et al. 2005). The longer the alkyl chain
of the cationic surfactants, the more enhancement of hydrolysis was observed in the
micelle system. When the maximum hydrolysis rate (k max ) in the micelle system
was used, the log( k max / k 2w ) versus log K s plot was linear, indicating the importance
of solubilization of (9) in the micelles. The retardation of alkaline hydrolysis in the
anionic micelles can be accounted for by an ionic repulsion of OH − from the
micelle surface, but the inhibitory mechanism by the nonionic micelles was not
clear. Similar hydrolytic profiles have been reported for dicarboximide pesticides
such as procymidone (10) (Villedieu et al. 1995). The reduced hydrolysis rates in
nonionic micelles were explained by the significant association between the pesti-
cides and micelles, and the polyethoxy chain might make the nucleophile OH − less
accessible to the carbonyl carbon of the pesticides. Similar interactions would play
a role in more favorable decarboxylation in the side chain of chlozolinate (18) after
ester cleavage in the nonionic micelles, and stabilization of the transition state
not involving water molecules was postulated. Similar hydrolytic resistance for
captan (47) and phosmet (66) having the dicarboximide moiety was reported for the
corresponding wettable powder formulations (Atwood et al. 1987).
Dehydrochlorination of DDT (1) under the alkaline conditions has been exam-
ined in the HDTMA Br micelles at a wide range of OH − concentration (Nome et al.
1982). At the lower OH − concentration of 10 −3 -10 −2 M, the micellar catalysis was
well described by the PPIE model, whereas additional reaction took place across
the interfacial boundary between the Stern and Gouy-Chapman layers of the
micelles at the higher OH − concentration (Stadler et al. 1984). Deviation from the
PPIE model is considered to originate from the very dynamic surface structure of
the micelles. Further, addition of a long-chain aliphatic alcohol such as hexanol was
found to reduce micellar catalysis in the HDTMA + micelles from that expected
from the PPIE model (Otero and Rodenas 1986). The incorporation of the long-
chain alcohol increased the volume of micelles, and the decrease of an effective
concentration would result in a reduced hydrolysis rate. Micellar catalysis by cati-
onic surfactants was observed for dicofol (20) mainly via the concentration effect,
which was confirmed by the positive value of an activation entropy (Nome et al.
1980). Inhibition of hydrolysis in the SDS micelles together with an insignificant